Solid lubricant and sliding member

ABSTRACT

A solid lubricant for being embedded in pores or grooves formed at a sliding surface of a sliding member body, comprises 5 to 30% by volume of a polyethylene resin, 20 to 60% by volume of a hydrocarbon-based wax and 10 to 60% by volume of melamine cyanurate. Such a solid lubricant can exhibit sliding properties identical to or higher than those of lead-containing solid lubricants, even under high load conditions.

TECHNICAL FIELD

The present invention relates to a solid lubricant and a sliding member,and more particularly, to a solid lubricant which is embedded in poresor grooves formed at a sliding surface of a sliding member body (a bodyportion of a sliding member), as well as a sliding member using thesolid lubricant.

BACKGROUND ART

Solid lubricants which are embedded in a sliding surface of a slidingmember such as bearings, form a thin film on the sliding surface, andexhibit a sliding effect. Therefore, a film-forming capability of thesolid lubricants has a large influence on friction coefficient and lifeof the resultant film. As such solid lubricants, there are known solidlubricants having a layer structure, in particular, those containinggraphite as a main component. The graphite exhibits a high resistingforce in the direction of load applied thereto, but has a low resistingforce in its sliding direction, owing to the layer structure. Inaddition, the graphite is a soft material and can maintain a goodlubrication performance over a broad temperature range of from anordinary temperature to a high temperature.

However, the solid lubricants containing graphite as a main componenttend to be not only insufficient in film-forming capability to someextent, but also unsatisfactory in life of the resultant film whenrepeatedly exposed to frictional contact. Therefore, the solidlubricants are unsuitable for use in high-load applications.

On the other hand, as the solid lubricants suitably used in high-loadapplications, there are known such solid lubricants prepared by blendinga polytetrafluoroethylene resin, a soft metal such as indium, lead andtin, and a wax with each other. In particular, there have been widelyused solid lubricants prepared by blending the ethylene tetrachlorideresin, lead and the wax with each other. Such solid lubricants exhibitan extremely low friction coefficient under high-load conditions and anexcellent film-forming capability, and a film obtained therefrom has along life and an excellent self-mending property.

In recent years, lead-free materials tend to be developed in theconsideration of avoiding environmental problems. This tendency of thematerial development has also been present in the field of solidlubricants. However, in the solid lubricants, lead is an importantconstituent for attaining satisfactory sliding properties. Inparticular, in the case where the solid lubricant are embedded in poresor grooves formed at the sliding surface of a sliding member such asbearings and used under high-load conditions, lead is important from thestandpoint of imparting a good film-forming capability thereto.

For example, in Japanese Patent Application Laid-Open (KOKAI) No.55-108427(1980), as a lead-free sliding member, there is described thesliding member produced by molding a resin containing an adduct ofmelamine and isocyanuric acid. However, in the case where the resincomposition for the sliding member containing the adduct of melamine andisocyanuric acid is used as a solid lubricant, the friction coefficientthereof tends to be insufficient under high-load conditions. Therefore,it has been demanded to provide a solid lubricant containing no lead andexhibiting sufficient sliding properties even under high-loadconditions.

DISCLOSURE OF THE INVENTION

The present invention has been conducted for solving the above problems.An object of the present invention is to provide a lead-free solidlubricant capable of exhibiting sliding properties which are identicalto or higher than those of conventional lead-containing solidlubricants, even under high-load conditions.

To accomplish the aim, in a first aspect of the present invention, thereis provided a solid lubricant comprising 5 to 30% by volume of apolyethylene resin, 20 to 60% by volume of a hydrocarbon-based wax and10 to 60% by volume of melamine cyanurate.

In a second aspect of the present invention, there is provided a slidingmember comprising a sliding member body having a sliding surface, andthe above solid lubricant which is embedded in pores and grooves formedat the sliding surface.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

The present invention is described in detail below. The polyethyleneresin used in the present invention has a function as a binder. Examplesof the polyethylene resin may include a high pressure-processedlow-density polyethylene (HPLD), a linear low-density polyethylene(LLDPE), a very low-density polyethylene (VLDPE), a high-densitypolyethylene (HDPE) or the like.

The high pressure-processed low-density polyethylene (HPLD) is anethylene homopolymer produced by a high pressure process, and containslong branched chains in addition to short branched chains such as ethylgroups. The density of the high pressure-processed low-densitypolyethylene is usually 0.910 to 0.940 g/cm³. The linear low-densitypolyethylene (LLDPE) is a copolymer of ethylene with the other α-olefinsuch as propylene, buten-1,4-methyl penten-1 and octen-1, which isproduced by a medium/low pressure process, and has a density of usually0.900 to 0.940 g/cm³. Of these copolymers, the polyethylenes having adensity of 0.925 to 0.940 g/cm³ are called a medium-density polyethylene(MDPE). The very low-density polyethylene (VLDPE) is obtained by furtherdecreasing the density of the linear low-density polyethylene (LLDPE),and has a density of 0.880 to 0.910 g/cm³. The high-density polyethylene(HDPE) is an ethylene homopolymer produced by a medium/low pressureprocess, and usually has a density of 0.940 to 0.970 g/cm³.

The amount of the polyethylene resin blended is 5 to 30% by volume,preferably 10 to 25% by volume. When the amount of the polyethyleneresin blended is less than 5% by volume, the polyethylene resin may failto exhibit a sufficient effect as a binder. When the amount of thepolyethylene resin blended is more than 30% by volume, the amounts ofthe hydrocarbon-based wax and the melamine cyanurate are comparativelysmall, so that it may be difficult to cause the resultant solidlubricant to show good sliding properties.

The hydrocarbon-based wax used in the present invention has an effectfor decreasing a friction coefficient. Examples of the hydrocarbon-basedwax may include paraffin waxes, polyethylene waxes, microcrystallinewaxes or the like. These hydrocarbon-based waxes may be used singly orin the form of a mixture of any two or more thereof.

The amount of the hydrocarbon-based wax blended is 20 to 60% by volume,preferably 25 to 45% by volume. When the amount of the hydrocarbon-basedwax blended is less than 20% by volume, the resultant solid lubricantmay fail to show the aimed low friction characteristics. On the otherhand, when the amount of the hydrocarbon-based wax blended is more than60% by volume, the resultant solid lubricant tends to be deteriorated inmoldability, and the molded product obtained therefrom tends to bedeteriorated in strength.

The melamine cyanurate used in the present invention is an adduct ofmelamine with cyanuric acid or isocyanuric acid, and has such astructure that melamine molecules and cyanuric acid (or isocyanuricacid) molecules respectively having a 6-membered ring structure arearranged in plane through a hydrogen bond to form overlapped layersbonded to each other by a weak force. Therefore, it is considered thatthe melamine cyanurate show a cleavage property similarly to molybdenumdisulfide or graphite. The melamine cyanurate has an effect of enhancinga wear resistance and a load carrying capacity. The amount of themelamine cyanurate blended is 10 to 60% by volume, preferably 20 to 50%by volume. When the amount of the melamine cyanurate blended is lessthan 10% by volume, it may be difficult to attain the aimed effect ofenhancing a wear resistance and a load carrying capacity. On the otherhand, when the amount of the melamine cyanurate blended is more than 60%by volume, the resultant solid lubricant tends to be deteriorated insliding properties.

The solid lubricant of the present invention may further contain, asadditional components, higher-fatty acids, esters of higher-fatty acids(higher-fatty esters), amides of higher-fatty acids (higher-fattyamides), metallic soaps, phosphates and/or high-molecular weightpolytetrafluoroethylene resins.

As the higher-fatty acids, there may be used saturated or unsaturatedaliphatic acids having 12 or more carbon atoms. Specific examples of thehigher-fatty acids may include lauric acid, myristic acid, palmiticacid, stearic acid, arachic acid, behenic acid, cerotic acid, montanicacid, melissic acid, lauroleic acid, myristoleic acid, oleic acid,elaidic acid, linoleic acid, linolenic acid, arachidonic acid, gadoleicacid and erucic acid.

The higher-fatty esters are esters of the above higher-fatty acids withmonohydric or polyhydric alcohols. Examples of the monohydric alcoholsmay include capryl alcohol, lauryl alcohol, myristyl alcohol, palmitylalcohol, stearyl alcohol and behenyl alcohol. Examples of the polyhydricalcohols may include ethylene glycol, propylene glycol, butane diol,glycerol, pentaerythritol and sorbitan. Specific examples of thehigher-fatty esters may include stearyl stearate, pentaerythritoltetrastearate, stearic monoglyceride, behenic monoglyceride and montanwaxes.

The higher-fatty amides are amides of the above higher-fatty acids withmonoamines or polyamines. Examples of the monoamines or polyamines mayinclude capryl amine, lauryl amine, myristyl amine, palmityl amine,stearyl amine, methylenediamine, ethylenediamine andhexamethylenediamine. Specific examples of the higher-fatty amides mayinclude stearamide, oleamide and erucamide.

These higher-fatty acids, higher-fatty esters and higher-fatty amideshave an effect of not only decreasing a friction coefficient but alsoenhancing a moldability, and may be used singly or in the form of amixture of any two or more thereof. The amount of the higher-fattyacids, higher-fatty esters and higher-fatty amides blended is usually 1to 10% by volume, preferably 3 to 10% by volume. When the amount of thehigher-fatty acids, higher-fatty esters and higher-fatty amides blendedis less than 1% by volume, it may be difficult to attain the aimedeffect of decreasing a friction coefficient and enhancing a moldability.When the amount of the higher-fatty acids, higher-fatty esters andhigher-fatty amides blended is more than 10% by volume, the resultantsolid lubricant tends to be deteriorated in moldability.

The metallic soap is a salt of the above higher-fatty acids with alkalimetals or alkali earth metals. Specific examples of the metallic soapmay include lithium stearate and calcium stearate. These metallic soapshave an effect of not only decreasing a friction coefficient but alsoenhancing a thermal stability. The amount of the metallic soap blendedis usually 3 to 20% by volume, preferably 5 to 15% by volume. When theamount of the metallic soap blended is less than 3% by volume, it may bedifficult to attain the aimed effect of decreasing a frictioncoefficient and enhancing a thermal stability. On the other hand, whenthe amount of the metallic soap blended is more than 20% by volume, theresultant solid lubricant tends to be deteriorated in moldability.

As the phosphate, there may be used tertiary phosphates, secondaryphosphates, pyrophosphates, phosphites or metaphosphates of alkalimetals or alkali earth metals. Specific examples of the phosphates mayinclude trilithium phosphate, dilithium hydrogenphosphate, lithiumpyrophosphate, tricalcium phosphate, calcium monohydrogenphosphate,calcium pyrophosphate, lithium metaphosphate, magnesium metaphosphateand calcium metaphosphate.

The phosphates themselves have no lubrication property, but can exhibitan effect of promoting formation of a lubricant film on the surface of amating member upon sliding thereon, so that it is possible to alwaysform and hold a good lubricant film on the surface of the mating member,thereby maintaining good sliding properties. The amount of the phosphateblended is usually 3 to 15% by volume, preferably 5 to 10% by volume.When the amount of the phosphate blended is less than 3% by volume, itmay be difficult to exhibit the aimed effect. On the other hand, whenthe amount of the phosphate blended is more than 15% by volume, theamount of the lubricant film transferred and attached onto the surfaceof the mating member tends to be excessively large, so that theresultant sliding member tends to be deteriorated in wear resistance.

In the case where at least two compound selected from the groupconsisting of the higher-fatty acids, higher-fatty esters, higher-fattyamides, metallic soaps and phosphates are used in combination, a totalamount of these components is preferably not more than 30% by volume.When the total amount of these components is more than 30% by volume,the resultant molded product tends to be insufficient in strength.

The high-molecular weight polytetrafluoroethylene resin is used mainlyfor molding purposes in the form of a molding powder or a fine powder,and represents such an polytetrafluoroethylene resin that is formed intofibers upon applying a shear force thereto (hereinafter referred tomerely as “PTFE”). The high-molecular weight PTFE may be directly usedin an unbaked state, or may be used in the form of particles obtained bybaking the PTFE at a temperature not less than a melting point thereofand then pulverizing the baked product. Specific examples of thehigh-molecular weight PTFE may include “TEFLON (registered trademark)7-J”, “TEFLON (registered trademark) 7A-J”, “TEFLON (registeredtrademark) 6-J” and “TEFLON (registered trademark) 6C-J”, all producedby Du Pont-Mitsui Fluorochemical Co., Ltd.; “POLYFLON M-12 (tradename)”and “POLYFLON F-201 (tradename)”, both produced by Daikin Industries,Ltd.; “FLUON G-163 (tradename)”, “FLUON G-190 (tradename)”, “FLUON CD076(tradename)” and “FLUON CD090 (tradename)”, all produced by Asahi GlassCo., Ltd.; “KT-300M (tradename)” produced by Kitamura Limited, or thelike. Further, in addition to the above products, there may also be usedPTFE modified with styrene-based polymers, acrylic ester-based polymers,methacrylic ester-based polymers, acrylonitrile-based polymers, etc.Specific examples of the modified PTFE may include “METABLEN A-3000(tradename)” produced by Mitsubishi Rayon Co., Ltd., or the like.

The high-molecular weight PTFE has an effect of enhancing a toughness ofthe solid lubricant. More specifically, the high-molecular weight PTFEcan effectively prevent the solid lubricant embedded in pores or groovesformed at the sliding surface of the sliding member body from beingbroken-off or fallen-off therefrom upon machining the solid lubricantembedded in the sliding member body (body portion of the sliding member)or upon using the sliding member. The amount of the high-molecularweight PTFE blended is usually 0.5 to 10% by volume, preferably 0.5 to5% by volume. When the amount of the high-molecular weight PTFE blendedis less than 0.5% by volume, it may be difficult to attain the aimedeffect of the high-molecular weight PTFE. On the other hand, when theamount of the high-molecular weight PTFE blended is more than 10% byvolume, the resultant solid lubricant tends to be deteriorated insliding properties and moldability.

The solid lubricant of the present invention can be produced by mixingthe above respective components with each other at a predeterminedmixing ratio using a mixer such as a Henschel mixer, a Super mixer, aball mill and a tumbler, and then molding the resultant mixture into adesired shape. The molding method is not particularly limited, and theremay be usually used such a molding method in which the obtained mixtureis fed to an extruder and melt-kneaded therein at a temperature capableof melting the hydrocarbon-based wax to prepare pellets, and then theobtained pellets are fed to an injection-molding machine andinjection-molded therefrom at a temperature not less than a softeningpoint of the polyethylene resin as a binder.

The sliding member of the present invention comprises a sliding memberbody composed of metal materials, etc., and a solid lubricant embeddedin pores or grooves formed at a sliding surface of the sliding memberbody. The solid lubricant may be fixed in the pores or grooves, forexample, using an adhesive agent.

The thus obtained solid lubricant can exhibit sliding properties thatare identical to or higher than those of conventional lead-containingsolid lubricants, even under high load conditions.

EXAMPLES

The present invention is described in more detail by Examples, but theExamples are only illustrative and not intended to limit the scope ofthe present invention. Various sliding properties used in Examples andComparative Examples were measured by the following methods.

<Sliding Properties>

The obtained solid lubricant was embedded in pores formed at a slidingsurface to prepare a sliding member test specimen. The thus obtainedtest specimen was subjected to a thrust test to measure a frictioncoefficient and a wear amount thereof. The testing conditions are shownin Table 1. TABLE 1 Material of test specimen High strength brassfourth- class casting Material of mating member Stainless steel (SUS304)Area ratio occupied by solid 30% lubricant in sliding surface Slidingvelocity 1 m/min Load 300 kgf/cm² Lubrication None Testing time 8 hr

Example 1

28% by volume of a linear low-density polyethylene “ULTZEX (tradename)”(produced by Mitsui Chemicals Inc.) as a polyethylene resin, 21% byvolume of a paraffin wax and 21% by volume of a polyethylene wax as ahydrocarbon-based wax, and 30% by volume of melamine cyanurate “MCA(tradename)” (produced by Mitsubishi Chemical Corporation) wereintroduced into a Henschel mixer and mixed with each other therein. Theresultant mixture was melt-kneaded using an extruder at a temperaturecapable of melting the hydrocarbon-based wax, thereby producing pellets.Next, the obtained pellets were fed into an injection-molding machineand injection-molded at a temperature not less than a softening point ofthe polyethylene resin, thereby producing a cylindrical solid lubricanthaving a diameter of 6 mm and a length of 5 mm. Various slidingproperties of the obtained solid lubricant are shown in Table 2.

Examples 2 to 20

The same procedure as defined in Example 1 was conducted except that thecomposition was variously changed as shown in Tables 2 to 8, therebyproducing a cylindrical solid lubricant having a diameter of 6 mm and alength of 5 mm. Various sliding properties of the obtained solidlubricants are shown in Tables 2 to 8.

Comparative Examples 1 to 3

The same procedure as defined in Example 1 was conducted except that thecomposition was variously changed as shown in Table 9, thereby producinga cylindrical solid lubricant having a diameter of 6 mm and a length of5 mm. Various sliding properties of the obtained solid lubricants areshown in Table 9. TABLE 2 Examples 1 2 3 Composition LLDPE 28 24 20 HDPE— — — HPLD — — — Paraffin wax 21 18 15 Polyethylene wax 21 18 15Melamine cyanurate 30 40 50 Montanic acid — — — Stearamide — — — Montanwax — — — Lithium stearate — — — Trilithium phosphate — — — Calciumpyrophosphate — — — Various properties Friction coefficient 0.110 0.1000.100 Wear amount (μm) 15 10 15

TABLE 3 Examples 4 5 6 Composition LLDPE 10 20 20 HDPE — — — HPLD — — —Paraffin wax 25 30 15 Polyethylene wax 25 — 15 Melamine cyanurate 40 4547 Montanic acid — 5 3 Stearamide — — — Montan wax — — — Lithiumstearate — — — Trilithium phosphate — — — Calcium pyrophosphate — — —Various properties Friction coefficient 0.065 0.075 0.090 Wear amount(μm) 3 3 7

TABLE 4 Examples 7 8 9 Composition LLDPE 20 20 20 HDPE — — — HPLD — — —Paraffin wax 15 15 15 Polyethylene wax 15 15 15 Melamine cyanurate 45 4035 Montanic acid 5 10 — Stearamide — — — Montan wax — — — Lithiumstearate — — 15 Trilithium phosphate — — — Calcium pyrophosphate — — —Various properties Friction coefficient 0.095 0.105 0.100 Wear amount(μm) 3 10 10

TABLE 5 Examples 10 11 12 Composition LLDPE 20 — 20 HDPE — 20 — HPLD — —— Paraffin wax 15 15 15 Polyethylene wax 15 15 15 Melamine cyanurate 4745 40 Montanic acid — 5 5 Stearamide — — — Montan wax — — — Lithiumstearate — — 5 Trilithium phosphate 3 — — Calcium pyrophosphate — — —Various properties Friction coefficient 0.100 0.100 0.090 Wear amount(μm) 10 15 11

TABLE 6 Examples 13 14 15 Composition LLDPE 20 20 — HDPE — — — HPLD — —20 Paraffin wax 15 15 15 Polyethylene wax 15 15 15 Melamine cyanurate 4035 35 Montanic acid 5 5 5 Stearamide — — — Montan wax — — — Lithiumstearate — 5 5 Trilithium phosphate 5 5 — Calcium pyrophosphate — — 5Various properties Friction coefficient 0.100 0.090 0.090 Wear amount(μm) 5 2 3

TABLE 7 Examples 16 17 Composition LLDPE 10 20 HDPE — — HPLD — —Paraffin wax 15 15 Polyethylene wax 35 15 Melamine cyanurate 30 20Montanic acid 5 5 Stearamide — 5 Montan wax — — Lithium stearate — 10Trilithium phosphate 5 10 Calcium pyrophosphate — — Various propertiesFriction coefficient 0.090 0.100 Wear amount (μm) 2 7

TABLE 8 Examples 18 19 20 Composition LLDPE — — — HDPE — — — HPLD 20 2020 Paraffin wax 12.5 12.5 12.5 Polyethylene wax 12.5 12.5 12.5 Melaminecyanurate 34 32 30 Montanic acid — — — Stearamide — — — Montan wax 5 5 5Lithium stearate 10 10 10 Trilithium phosphate 5 5 5 Calciumpyrophosphate — — — High-molecular weight 1 3 5 PTFE Various propertiesFriction coefficient 0.080 0.090 0.100 Wear amount (μm) 2 4 7

TABLE 9 Comparative Examples 1 2 3 Composition LLDPE 50 50 — HDPE — — —HPLD — — 10 Paraffin wax 50 — 13 Polyethylene wax — — — Melaminecyanurate — 50 — Montanic acid — — — Stearamide — — — Montan wax — — —Lithium stearate — — 7 Trilithium phosphate — — — Calcium pyrophosphate— — — Lead — — 40 Low-molecular weight — — 30 PTFE Various propertiesFriction coefficient 0.145 >0.2 0.095 Wear amount (μm) 25 — 9

In the above Tables, as HDPE, there was used “HI-ZEX (tradename)”produced by Mitsui Chemicals Inc.; as HPLD, there was used “MIRASON(tradename)” produced by Mitsui Chemicals Inc.; as high-molecular weightPTFE, there was used “KT-300M (tradename)” produced by Kitamura Limited;and as low-molecular weight PTFE, there was used “TLP-10F (tradename)”produced by Du Pont-Mitsui Fluorochemical Co., Ltd.

As apparently recognized from the above results, it was confirmed thatthe sliding member obtained by embedding the solid lubricant of thepresent invention in the sliding member body exhibited excellent slidingproperties that were identical to or higher than those of the slidingmember in which the conventional lead-containing solid lubricantobtained in Comparative Example 3 was embedded. On the other hand, itwas confirmed that the sliding member having a sliding surface in whichthe solid lubricant containing no melamine cyanurate as obtained inComparative Example 1 was embedded, exhibited a high frictioncoefficient, a large wear amount and poor sliding properties; and thesliding member having a sliding surface in which the solid lubricantcontaining no hydrocarbon-based wax as obtained in Comparative Example 2was embedded, exhibited a friction coefficient as high as more than 0.2in the course of the thrust test, resulting interruption of the thrusttest.

INDUSTRIAL APPLICABILITY

According to the present invention, there can be provided a lead-freesolid lubricant capable of exhibiting sliding properties identical to orhigher than those of conventional lead-containing solid lubricants, evenunder high load conditions, as well as a sliding member using the solidlubricant.

1. A solid lubricant for being embedded in pores or grooves formed at asliding surface of a sliding member body, comprising 5 to 30% by volumeof a polyethylene resin, 20 to 60% by volume of a hydrocarbon-based waxand 10 to 60% by volume of melamine cyanurate.
 2. A solid lubricantaccording to claim 1, wherein said hydrocarbon-based wax is at least onematerial selected from the group consisting of paraffin waxes,polyethylene waxes and microcrystalline waxes.
 3. A solid lubricantaccording to claim 1, further comprising as an additional component, atleast one compound selected from the group consisting of higher-fattyacids, higher-fatty esters and higher-fatty amides in an amount of 1 to10% by volume.
 4. A solid lubricant according to claim 1, furthercomprising as an additional component, a metallic soap in an amount of 3to 20% by volume.
 5. A solid lubricant according to claim 1, furthercomprising as an additional component, a phosphate in an amount of 3 to15% by volume.
 6. A solid lubricant according to claim 1, furthercomprising as an additional component, a high-molecular weightpolytetrafluoroethylene resin in an amount of 0.5 to 10% by volume.
 7. Asliding member comprising a sliding member body, and the solid lubricantas defined in claim 1, which is embedded in pores and grooves formed atthe sliding surface of the sliding member body.